Printed wiring board, method for forming the printed wiring board, and board interconnection structure

ABSTRACT

A board interconnection structure having a first printed wiring board in which a first conductive circuit is arranged on a first insulating layer, the first conductive circuit having, on an end portion thereof, a first connection terminal in which an upper surface width is narrower than a bottom surface width; a second printed wiring board in which a second conductive layer having a second connection terminal is arranged on a second insulating layer; and a connection layer that forms fillets along longitudinal side surfaces of the first connection terminal, and interconnects the first connection terminal and the second connection terminal. The first connection terminal may have a projection portion.

Priority is claimed from Japanese Patent Application No. 2006-145389 and2006-145390, filed May 25, 2006, the contents of which are incorporatedherein by reference.

BACKGROUND

The present invention relates to a technology for interconnectingprinted boards. In particular, the present invention relates to aprinted wiring board, a method for forming the printed wiring board, anda board interconnection structure, which enhances interconnectionstrength of the boards.

DESCRIPTION OF THE RELATED ART

As an electronic instrument is being required to be smaller, lighter inweight, and higher in function, it is more necessary to install aplurality of boards three-dimensionally in a small space of the product.For this, a space for connecting electric signals between the boardsmust be reduced. However, if the function of the electronic instrumentbecomes higher, types of the electric signals between the boards arealso increased, and the above-described connecting space is increased,which lead to inhibit such miniaturization and weight reduction. Forinterconnection of the boards, it is common to use connector parts.However, it is difficult to miniaturize the connector parts havingfitting mechanisms. Moreover, in the connector parts, electric bondingis performed therefor by compressively bonding terminals to each other,and accordingly, the connector parts are inferior in bonding themselvesis generated, and accordingly, expense of the connector parts is addedto total cost in the case of multi-signal connection.

In this connection, in recent years, in the case of electricallyinterconnecting printed wiring boards such as a rigid board and aflexible board, and in particular, in the case of electricallyinterconnecting narrow-pitch wires of these boards, a connection methodby solder connection is employed as a method that does not use theconnector parts. Specifically, connection terminal portions of a pair ofprinted wiring boards are mutually soldered. A description will be madebelow of the connection method by the soldering by using FIGS. 1A and1B.

FIG. 1A and FIG. 1B show a structure of a connection portion when arigid wiring board 101 and a flexible wiring board 102 areinterconnected by the solder wiring. FIG. 1A shows a lateral crosssection of the connection portion, and FIG. 1B shows a longitudinalcross section (cross section along a line 1B-1B of FIG. 1A) of theconnection portion. The flexible wiring board 102 includes a flexibleinsulating layer 103, a conductive circuit 104 provided on the flexibleinsulating layer 103, connection terminals 104 a as spots of theconductive circuit 104, which are subjected to the solder connection,and a flexible insulating protective layer 105 that protects theconductive circuit 104. The rigid wiring board 101 includes aninsulating layer 106, a conductive circuit 107 provided on theinsulating layer 106, connection terminals 107 a as spots of theconductive circuit 107, which are subjected to the solder connection,and a flexible insulating protective layer 108 that protects theconductive circuit 107.

With regard to a method for supplying a solder 109 between theconnection terminals 104 a and 107 a, solder plating is implemented forsurfaces of both of the connection terminals 104 a and 107 a or forsurfaces of either thereof, or alternatively, a cream solder is printedon the surfaces of the connection terminals 107 a. After the solder 109is supplied between the connection terminals 104 a and 107 a, theconnection terminals 104 a and 107 a are made to face each other, andthe rigid wiring board 101 and the flexible wiring board 102 arepositionally aligned and stacked on each other. While keeping thisstate, the entirety of the connection portions are heated up by a heatersuch as a heater chip until the solder 109 is molten. Then, as shown inFIG. 1B, the connection terminals 104 a and 107 a are interconnected. Asa result, electric conduction between the rigid wiring board 101 and theflexible wiring board 102 is made possible.

Since, however, microfabrication and pitch fining of the conductivecircuits of the printed wiring boards are advanced in recent years,problems occur in the connection method as described above.Specifically, in the connection structure shown in FIGS. 1A and 1B, themolten solder 109 squeezes out if the solder 109 is excessively appliedbetween the connection terminals 104 a and 107 a, when the connectionterminals 104 a and 107 a are interconnected by thermocompressionbonding using the solder 109. Therefore, there is an apprehension thatthe solder that has squeezed out may be brought into contact with thesolder on the adjacent terminals, and may form unexpected solder bridgesbetween the connection terminals.

Accordingly, as a proposal against this problem, Japanese PatentLaid-Open Publication No. H8-23147 shows connection terminals on theflexible wiring board, which are formed to be narrower in width thanconnection terminals on the opposite circuit board. According to this,the connection terminals on the flexible wiring board are arrangedwithin the width of the connection terminals of the circuit board, andsolder fillets are formed along a longitudinal direction on theconnection terminals of the circuit board. As a result, the solder isprevented from flowing out to regions of the adjacent connectionterminals. In the above-described proposal, however, connection strengthbetween the connection terminals and the flexible insulating layerbecomes a problem. In the case of comparing connection strength when theconnection terminals are bonded to each other by the solder withconnection strength between the connection terminals and the flexibleinsulating layer, the former connection strength is higher than thelatter one since metal bonding is formed in the former one. Hence, inthe case of considering the connection strength, the connection betweenthe connection terminals and the flexible insulating layer becomesimportant. In this case, the wider connection terminal is moreadvantageous. However, since the connection terminals on one side arethinned in the above-described proposal, the connection strength againsta stress in a tensile direction or a peeling direction is weakened.

Moreover, the above-described proposal has a harmful effect on themicrofabrication of the connection portions. In the printed wiringboards, limitations are imposed on a width of processable conductors anda minimum value of a pitch therebetween. In the above-describedproposal, it becomes necessary to widen the width of the connectionterminals on at least one side more than the minimum width at which itis possible to process the conductors. This inhibits the microconnection portions from being realized.

In this connection, Japanese Patent Laid-Open Publication No. H9-46031proposes to form slits on the connection terminals on the flexiblewiring board in order to increase the above-described connectionstrength, that is, tensile strength or peeling strength between theboards. According to this publication, the excessive solder is stored inthe slits. Hence, short circuit owing to the excessive solder isprevented, and in addition, the connection strength is enhanced byfillets formed on both sides of the slits.

It is considered to use etching treatment as means for forming the slitson the connection terminals. However, there are limitations onmicrofabrication of the circuits in terms of the width and the slits(i.e. circuit interval), which can be formed by the etching treatment.For example, when one slit is formed on the center of each connectionterminal 104 a shown in FIG. 1B, and each connection terminal 104 a isdivided into two, it is necessary that the circuit width necessary forthe connection portions be set equal to or more than a width obtained byadding a width of the formable slits to a width at least double a widthof the formable circuits. That is, it is necessary to thicken thecircuit width. This inhibits the pitch between the circuits from beingmade more micro. Moreover, when the slits are formed on the connectionterminals on the conductive circuits on one side, an area where theconnection terminals are pasted on the insulating layer is reduced.Therefore, when a stress that peels the flexible wiring board from therigid wiring board is applied thereto, the peeling becomes prone tooccur on interfaces between the connection terminals and the insulatinglayer, resulting in reduction of the connection strength as a whole.

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION

It is an aspect of the present invention to provide a printed wiringboard, a method for forming the printed wiring board, and a boardinterconnection structure, which can prevent the decrease of the peelingstrength between the connection terminals and the insulating layer andprevent the short circuit by storing the excessive solder withoutinhibiting the pitch between the circuits from being made more micro,thereby enhance the connection strength between the connectionterminals.

A first aspect of the present invention is a printed wiring board,including: an insulating layer; and a conductive circuit arranged on theinsulating layer, the conductive circuit having, on an end portionthereof, a connection terminal in which an upper surface width isnarrower than a bottom surface width.

Here, the connection terminal may be formed by providing thereon aprojection portion in a direction where the conductive circuit isextended.

A second aspect of the present invention is a board interconnectionstructure, including: a first printed wiring board in which a firstconductive circuit is arranged on a first insulating layer, the firstconductive circuit having, on an end portion thereof, a first connectionterminal in which an upper surface width is narrower than a bottomsurface width; a second printed wiring board in which a secondconductive circuit having a second connection terminal is arranged on asecond insulating layer; and a connection layer that forms fillets alonglongitudinal side surfaces of the first connection terminal, andinterconnects the first connection terminal and the second connectionterminal.

Here, in the above-described interconnection structure, a projectionportion may be provided on the first connection terminal of the firstconductive circuit in a direction where the first conductive circuit isextended.

A third aspect of the present invention is a method for forming aprinted wiring board, including: preparing an insulating layer, on asurface of which a conductive circuit having a connection terminal isarranged; coating resist on the insulating layer and the conductivecircuit; patterning the resist into a desired pattern; by using thepatterned resist, forming a projection portion on the conductive circuitin a direction where the conductive circuit is extended; and removingthe patterned resist.

According to the present invention, the decrease of the peeling strengthbetween the connection terminals and the insulating layer, and thesolder bridges (short circuit) between the connection terminals, owingto excessive solder, can be prevented without inhibiting the pitchbetween the circuits from being made more micro. Hence, the printedwiring board, the method for forming the printed wiring board, and theboard interconnection structure, which enhance the connection strengthbetween the connection terminals, can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view of connection portions of aconventional board interconnection structure.

FIG. 1B is a cross-sectional view when the board interconnectionstructure of FIG. 1A is viewed from a direction of a line 1B-1B of FIG.1A.

FIG. 2 is a plan view of a printed wiring board according to a firstnon-limiting embodiment of the present invention.

FIG. 3 is a cross-sectional view when the printed wiring board of FIG. 2is viewed from a direction of a line 3-3 of FIG. 2.

FIG. 4 is a cross-sectional view of a board interconnection structureaccording to the first non-limiting embodiment.

FIG. 5 is a cross-sectional view of a board interconnection structureaccording to a second non-limiting embodiment.

FIG. 6 is a plan view of a printed wiring board according to a thirdnon-limiting embodiment.

FIG. 7 is a cross-sectional view when the printed wiring board of FIG. 6is viewed from a direction of a line 7-7 of FIG. 6.

FIG. 8A to FIG. 8E are cross-sectional views of manufacturing steps,showing a method for forming the printed wiring board according to thethird non-limiting embodiment.

FIG. 9 is a cross-sectional view of a board interconnection structureaccording to the third non-limiting embodiment.

FIG. 10A and FIG. 10B are cross-sectional views of manufacturing steps,showing a connection method of the board interconnection structureaccording to the third non-limiting embodiment.

FIG. 11A to FIG. 11E are cross-sectional views of manufacturing steps,showing a method for forming a printed wiring board according to afourth non-limiting embodiment.

FIG. 12A is a cross-sectional view of a printed wiring board accordingto a fifth non-limiting embodiment.

FIG. 12B is a cross-sectional view of a board interconnection structureaccording to the fifth non-limiting embodiment.

FIG. 13 is a plan view of a printed wiring board according to othernon-limiting embodiments.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS OF THE INVENTION

A description will be made below of non-limiting embodiments of thepresent invention with reference to the drawings. In the followingdescription, the same or similar portions of the drawings are denoted bythe same or similar reference numerals. Note that the drawings areschematic, and that relationships between thicknesses and planardimensions, ratios of thicknesses of the respective layers, and the likediffer from the actual ones. Hence, specific thicknesses and dimensionsshould be determined by referring to the following description.Moreover, it is a matter of course in the drawings, that portions inwhich the dimensional relationships and the ratios are mutuallydifferent are included.

First Non-Limiting Embodiment (Printed Wiring Board)

As shown in FIG. 2, a printed wiring board according to a firstembodiment of the present invention includes an insulating layer 10, andconductive circuits 11 having, on end portions thereof, connectionterminals 11 a in each of which a width W₁ of an upper surface isnarrower than a width W₂ of a bottom surface. FIG. 3 is across-sectional view of the printed wiring board viewed from a directionof a line 3-3 of FIG. 3.

As the insulating layer 10, for example, a flexible board such as apolyimide board, a polyethylene terephthalate (PET) board, and apolyethylene naphthalate (PEN) board can be used. Alternatively, as theinsulating layer 10, for example, a hard rigid board such as a glassepoxy board, a glass composite board, and a paper epoxy board can beused. It is preferable that the insulating layer 10 have heat resistanceto a temperature of a melting point of a solder or higher. In the caseof using the rigid board as the insulating layer 10, for a thicknessthereof, 2.4 mm, 2.0 mm, 1.6 mm. 1.2 mm, 1.0 mm, 0.8 mm, 0.6 mm, 0.4 mm,0.2 mm, and the like can be employed. Moreover, in the case of using theflexible board as the insulating layer 10, for a thickness thereof, 25μm, 12.5 μm, 8 μm, 6 μm, and the like can be employed.

The conductive circuits 11 form a circuit pattern of conductors, whichare designed on the insulating layer 10. On the insulating layer 10, theconductive circuits 11 are formed of rolled copper foil, electrolyticcopper foil, or the like by pattern processing. In the conductivecircuits 11, metal foil other than the copper foil is also usable as theconductors. A pitch between the conductors in the conductive circuit 11is set at 10 to 500 μm, and a width of the conductors is set at 10 to500 μm. For a thickness of the conductive circuits 11, 35 μm, 18 μm, 12μm, 9 μm, and the like can be employed. On the conductive circuits 11,coverlay films that use, as a base material, an insulating polyimidefilm having excellent flexibility even after being adhered, or the likeare arranged as cover layers (not shown).

The connection terminals 11 a are formed by a subtractive method, thusmaking it possible to be formed so that the upper surface width W₁ canbe set smaller than the bottom surface width W₂. When the insulatinglayer 10 is the flexible board, the connection terminals 11 a can bearranged so as to be extended to an end portion of the insulating layer10. Meanwhile, when the insulating layer 10 is the rigid board, it ispreferable that the connection terminals 11 a be arranged so as to keepa little space from the end portion of the insulating layer 10. Theconnection terminals 11 a are subjected to surface treatment by prefluxtreatment, hot air leveling (HAL), electrolytic solder plating,electroless solder plating, and the like.

According to the above-described printed wiring board, a connection areabetween the insulating layer 10 and the connection terminals 11 a is notreduced, and accordingly, connection strength between the insulatinglayer 10 and the connection terminals 11 a is not decreased. Moreover,the upper surface width W₁ of each connection terminal 11 a is narrowerthan the bottom surface width W₂ thereof, thus making it possible toform fillets 23 (shown in FIG. 4) along longitudinal side surfaces ofthe connection terminals 11 a. Accordingly, formation of solder bridgesand a connection failure in the connection layers 19 (shown in FIG. 4)can be prevented.

Moreover, according to the above-described printed wiring board, theconnection terminals 11 a may be formed so as to have the minimum widthat which the connection terminals 11 a can be processed. Accordingly,micro connection portions can be realized.

Furthermore, the connection terminals 11 a can be formed by thesubtractive method, and accordingly, when the conductive circuits 11 areprocessed by the subtractive method, the conductive circuits 11 can beformed without increasing the number of manufacturing steps.

(Board Interconnection Structure)

As shown in FIG. 4, a board interconnection structure according to thefirst embodiment of the present invention includes: a first printedwiring board 1 in which first conductive circuits are arranged on thefirst insulating layer 10, the first conductive circuits having, on endportions thereof, the first connection terminals 11 a in which the uppersurface width W_(i) is narrower than the bottom surface width W₂; asecond printed wiring board 2 in which second conductive circuits havingsecond connection terminals 13 a are arranged on a second insulatinglayer 12; and connection layers 19 in which the fillets 23 are formedalong the longitudinal side surfaces of the first connection terminals11 a. The connection layers 19 interconnects the first connectionterminals 11 a and the second connection terminals 13 a.

As the second insulating layer 12, for example, the hard rigid boardsuch as the glass epoxy board, the glass composite board, and the paperepoxy board can be used. Moreover, the flexible board can also be usedas the second insulating layer 12. It is preferable that the secondinsulating layer 12 have the heat resistance to the temperature of themelting point of the solder or higher. In the case of using the rigidboard, for a thickness thereof, 2.4 mm, 2.0 mm, 1.6 mm. 1.2 mm, 1.0 mm,0.8 mm, 0.6 mm, 0.4 mm, 0.2 mm, and the like can be employed. Moreover,in the case of using the flexible board, for a thickness thereof, 25 μm,12.5 μm, 8 μm, 6 μm, and the like can be employed.

The second conductive circuits form a circuit pattern of conductors,which are designed on the second insulating layer 12. The secondconductive circuits are formed by performing the pattern processing forthe rolled copper foil or the electrolytic copper foil on the secondinsulating layer 12. For the second conductive circuits, the metal foilother than the copper foil is also usable. A pitch between theconductors in the second conductive circuit is set at 10 to 500 μm, anda width of the conductors is set at 10 to 500 μm. For a thickness of thesecond conductive circuits, 35 μm, 18 μm, 12 μm, 9 μm, and the like canbe employed. On the second conductive circuits, the coverlay films orthe like are arranged as cover layers (not shown). In the case of usingthe rigid board, the coverlay films use solder resist as a basematerial, and in the case of using the flexible board, the coverlayfilms use, as the base material, the insulating polyimide film havingthe excellent flexibility even after being adhered, or the like.

A width of the second connection terminals 13 a is set substantiallyequal to the bottom surface width W₂ of the first connection terminals11 a. A thickness of the second connection terminals 13 a can be set,for example, at 15 μm. When the second insulating layer 12 is theflexible board, the second connection terminals 13 a can be arranged soas to be extended to an end portion of the second insulating layer 12.Meanwhile, when the second insulating layer 12 is the rigid board, it ispreferable that the second connection terminals 13 a be arranged so asto keep a little space from the end portion of the second insulatinglayer 12. The second connection terminals 13 a are subjected to thesurface treatment by the preflux treatment, the hot air leveling (HAL),the electrolytic solder plating, the electroless solder plating, and thelike.

A description will be made of a connection method of the boardinterconnection structure according to the first embodiment.

First, at least either of the first connection terminals 11 a and thesecond connection terminals 13 a is applied with a solder paste orsubjected to the solder plating, whereby a solder with a thickness ofapproximately 3 μm is disposed thereon. Then, the first connectionterminals 11 a and the second connection terminals 13 a are arranged soas to face each other. Moreover, these boards are heated up to 200° C.or higher by the heater such as the heater chip, whereby the connectionlayers 19 are formed therebetween. As a result, the first connectionterminals 11 a and the second connection terminals 13 a are bonded toeach other. When the solder is molten to form the connection layers 19,the solder is stored along the longitudinal side surfaces of the firstconnection terminals 11 a, which are gaps between the first connectionterminals 11 a and the second connection terminals 13 a, thereby formingthe fillets 23. Epoxy underfill resin or the like (not shown) is filledinto peripheries of the connection layers 19 by using a capillaryphenomenon. The underfill resin is filled there, thus making it possibleto increase the connection strength of the connection layers 19, and toprevent the solder from flowing into the adjacent connection terminals.By the manufacturing steps described above, the board interconnectionstructure shown in FIG. 3 is formed. Note that, as a bonding materialfor use in the connection layers 19, a lead-containing solder paste, alead-free solder paste, solder plating, tin plating, and the like can beused.

According to the board interconnection structure described above, theconnection area between the first insulating layer 10 and the firstconnection terminals 11 a is not reduced, and accordingly, theconnection strength between the first insulating layer 10 and the firstconnection terminals 11 a is not decreased. Moreover, the upper surfacewidth W₁ of each first connection terminal 11 a is narrower than thebottom surface width W₂ thereof, thus making it possible to form thefillets along the longitudinal side surfaces of the first connectionterminals 11 a. Accordingly, the formation of the solder bridges and theconnection failure in the connection layers 19 can be prevented.

Moreover, according to the above-described board interconnectionstructure, both of the first connection terminals 11 a and the secondconnection terminals 13 a may be formed so as to have the minimum widthsat which both can be processed. Accordingly, the micro connectionportions can be realized.

Furthermore, the bottom surface width W₂ of the first connectionterminals 11 a and the width of the second connection terminals 13 a areset substantially equal to each other, whereby both of the connectionterminals 11 a and 13 a do not decrease the connection strength betweenthe first insulating layers 10 and the second insulating layer 12.

Second Non-Limiting Embodiment

As shown in FIG. 5, a board interconnection structure according to asecond embodiment of the present invention is different from that ofFIG. 4, in the case of comparison therebetween, in that an upper surfacewidth W₃ of the connection terminals 13 b is more narrow than a bottomsurface width W₄ of the connection terminals 13 b of a second printedwiring board 2 a. Other elements shown in FIG. 5 are substantiallysimilar to those in the board interconnection structure shown in FIG. 4,and accordingly, a duplicate description will be omitted.

In accordance with the board interconnection structure according to thesecond embodiment, connection areas between the first insulating layer10 and the first connection terminals 11 a and between the secondinsulating layer 12 and the second connection terminals 13 b are notreduced. Hence, the connection strengths between the first insulatinglayer 10 and the first connection terminals 11 a and between the secondinsulating layer 12 and the second connection terminals 13 b are notdecreased. Moreover, the upper surface width W₁ of the first connectionterminals 11 a is narrower than the bottom surface width W₂ thereof,thus making it possible to form the fillets along the longitudinal sidesurfaces of the first connection terminals 11 a. Therefore, theformation of the solder bridges and the connection failure in connectionlayers 19 a can be prevented. In a similar way, the upper surface widthW₃ of the second connection terminals 13 b is narrower than the bottomsurface width W₄ thereof, thus making it possible to form fillets alonglongitudinal side surfaces of the second connection terminals 13 b.Hence, the formation of the solder bridges and the connection failure inthe connection layers 19 a can be prevented.

Moreover, in accordance with the board interconnection structureaccording to the second embodiment, both of the first connectionterminals 11 a and the second connection terminals 13 b may be formed soas to have the minimum widths at which both can be processed.Accordingly, the micro connection portions can be realized.

Furthermore, the bottom surface width W₂ of the first connectionterminals 11 a and the bottom surface width W₄ of the second connectionterminals 13 b are set substantially equal to each other, whereby bothof the connection terminals 11 a and 13 b do not decrease the connectionstrength between the first insulating layers 10 and the secondinsulating layer 12.

Note that, in the first and second embodiments, each of the crosssections of the first connection terminals 11 a and the secondconnection terminals 13 b forms a trapezoidal shape in which sidesurfaces are linear; however, the side surfaces may be bent in an arcshape. If the side surfaces are bent in the arc shape, whereby surfaceareas of the side surfaces of the first connection terminals 11 a andthe second connection terminals 13 b are increased, then connectionareas of these connection terminals to the solder are increased, thusmaking it possible to enhance the connection strength therebetween.

Third Non-Limiting Embodiment (Printed Wiring Board)

As shown in FIG. 6, a printed wiring board according to a thirdembodiment of the present invention includes an insulating layer 10,conductive circuits 14 arranged on the insulating layer 10 and havingconnection terminals 15 on end portions thereof, and projection portions16 provided on the connection terminals 15 in a direction where theconductive circuits 14 are extended. FIG. 7 is a cross-sectional view ofthe printed wiring board viewed from a direction of a line 7-7 of FIG.6.

The insulating layer 10 is similar to that of the first embodiment.

The conductive circuits 14 form a circuit pattern of conductors, whichare designed on the insulating layer 10. In the case of the patternformation using the subtractive method, the conductive circuits 14 areformed by etching the rolled copper foil, the electrolytic copper foil,or the like on the insulating layer 10. Other metal foil than the copperfoil is also usable as the conductors. For a thickness of the conductivecircuits 14, 35 μm, 18 μm, 12 μm, 9 μm, 5 μm, and the like can beemployed. When the conductive circuits 14 are formed by the subtractivemethod, the minimum pitch between the conductive circuits 14 becomes 35μm in consideration that a width of the circuits is 15 μm at theminimum, and that a space width between the circuits is 20 μm at theminimum. In the case of a semi-additive method of forming the conductivecircuits 14 on the insulating layer 10 by plating, the minimum pitchbetween the circuits becomes 20 μm in consideration that the width ofthe circuits is 10 μm at the minimum, and that the space width betweenthe circuits is 10 μm at the minimum. Specifically, the semi-additivemethod enables the microfabrication more than the subtractive method. Onthe conductive circuits 14, as the cover layers (not shown), there arearranged the coverlay films or the like, which use, as the basematerial, the insulating polyimide film having the excellent flexibilityafter being adhered, or the like.

A thickness of the connection terminals 15 can be set, for example, at15 μm and 8 μm. When the insulating layer 10 is the flexible board, theconnection terminals 15 can be arranged so as to be extended to the endportion of the insulating layer 10. Meanwhile, when the insulating layer10 is the rigid board, it is preferable that the connection terminals 15be arranged so as to keep a little space from the end portion of theinsulating layer 10.

As shown in FIG. 7, the projection portions 16 are provided on theconnection terminals 15. A width of the projection portions 16, that is,a width thereof in a perpendicular direction to the direction where theconductive circuits 14 are extended is set, for example, at 75% of thewidth of the connection terminals 15. When the width of the connectionterminals 15 is 20 μm, the width of the projection portions 16 becomes15 μm (=20 μm×0.75). With regard to a thickness of the projectionportions 16, it is preferable that the sum of the thickness concernedand the thickness of the connection terminals 15 be equivalent to thethickness of the connection terminals of the conventional printed wiringboard. For example, the thickness of the projection portions 16 can beset at 10 μm. A material of the projection portions 16 may be the samematerial as that of the conductive circuits 14, or alternatively, may bea different material from that of the conductive circuits 14 as long asa melting point of the material is higher than the melting point of thesolder. As the material of the projection portions, there can be usedcopper (Cu) plating, nickel (Ni) plating, gold (Au) plating, Ni/Auplating in which the Au plating is further formed on the Ni plating, andthe like. For example, a thickness of the Ni/Au plating becomesapproximately the sum of the thickness (2 to 8 μm) of the Ni plating andthe thickness (0.03 μm) of the Au plating. In the event of selecting thematerial of the projection portion 16, electric characteristics(conduction resistance, migration characteristics), mechanical strength,and controllability for the shape of the plating become important.

In accordance with the printed wiring board according to the thirdembodiment, the connection area between the insulating layer 10 and theconnection terminals 15 is not reduced. Hence, peeling strength betweenthe insulating layer 10 and the connection terminals 15 is notdecreased. Moreover, the projection portions 16 are provided, whereby asurface area of each connection terminal 15 is increased. Therefore, theconnection strength can be enhanced.

(Method for Forming Printed Wiring Board)

A description will be made below of a non-limiting method for forming aprinted wiring board according to the third non-limiting embodimentwhile referring to FIG. 8.

(I) First, the insulating layer 10, on which the conductive circuitshaving the connection terminals 15 are arranged, is prepared. Then,resist 30 is formed on the insulating layer 10 and the connectionterminals 15 (refer to FIG. 8A). As the resist 30, for example,photocuring photoresist can be used.

(II) Next, spots on which the projection portions 16 will not be formedare irradiated with light and exposed, whereby the resist 30 isdenatured so as to cure, and is formed into cured resist 32 (refer toFIG. 8B). Then, the resist 30 that has not turned to the cured resist 32is removed by an alkaline solution and the like (refer to FIG. 8C). Insuch a way, the resist 30 is patterned into a desired pattern.

(III) Next, the plating is performed by using the cured resist 32 thuspatterned, thus making it possible to form the projection portions 16 onthe connection terminals 15 in the direction where the conductivecircuits are extended (refer to FIG. 8D).

(IV) Next, the cured resist 32 thus patterned is removed (refer to FIG.8E).

By the above-described manufacturing steps, the printed wiring boardaccording to the third embodiment is formed.

In accordance with the method for forming a printed wiring boardaccording to the third embodiment, the projection portions 16 are formedby the plating, thus making it possible to microfabricate the projectionportions 16. Moreover, since the plating is performed by using the curedresist 32 that is patterned, it is easy to select and use a suitableplating material.

(Board Interconnection Structure)

As shown in FIG. 9, the board interconnection structure according to thethird embodiment of the present invention includes: the first printedwiring board 1 in which the first conductive circuits having the firstconnection terminals 15 on the end portions thereof are arranged on thefirst insulating layer 10, and the projection portions 16 are providedon the first connection terminals 15 in the direction where the firstconductive circuits are extended; the second printed wiring board 2 inwhich second conductive circuits having second connection terminals 22are arranged on a second insulating layer 20; and connection layers 42which form the fillets on side surface portions of the projectionportions 16 and interconnect the first connection terminals 15 and thesecond connection terminals 22.

As the second insulating layer 20, for example, the hard rigid boardsuch as the glass epoxy board, the glass composite board, and the paperepoxy board can be used. Moreover, the flexible board can also be usedas the second insulating layer 20. In the case of using the rigid board,for a thickness thereof, 2.4 mm, 2.0 mm, 1.6 mm. 1.2 mm, 1.0 mm, 0.8 mm,0.6 mm, and the like can be employed. Moreover, in the case of using theflexible board, for a thickness thereof, 25 μm, 12.5 μm, 8 μm, 6 μm, andthe like can be employed.

The second conductive circuits form a circuit pattern of conductors,which are designed on the second insulating layer 20. The secondconductive circuits are formed by performing the pattern processing forthe rolled copper foil or the electrolytic copper foil on the secondinsulating layer 20. For the second conductive circuits, the metal foilother than the copper foil is also usable. For a thickness of the secondconductive circuits, 35 μm, 18 μm, 12 μm, 9 μm, and the like can beemployed. When the second conductive circuits are formed by thesubtractive method, the minimum pitch between the second conductivecircuits becomes 35 μm in consideration that a width of the circuits is15 μm at the minimum, and that a space width between the circuits is 20μm at the minimum. Meanwhile, when the second conductive circuits areformed by the semi-additive method, the minimum pitch between the secondcircuits becomes 20 μm in consideration that the width of the circuitsis 10 μm at the minimum, and that the space width between the circuitsis 10 μm at the minimum. On the second conductive circuits, the coverlayfilms or the like are arranged as the cover layers (not shown). In thecase of using the rigid board, the coverlay films use the solder resistas a base material, and in the case of using the flexible board, thecoverlay films use, as the base material, the insulating polyimide filmhaving the excellent flexibility even after being adhered, or the like.

A thickness of the second connection terminals 22 can be set, forexample, at 15 μm and 8 μm. When the insulating layer 20 is the flexibleboard, the second connection terminals 22 can be arranged so as to beextended to the end portion of the second insulating layer 20.Meanwhile, when the second insulating layer 20 is the rigid board, it ispreferable that the second connection terminals 22 be arranged so as tokeep a little space from the end portion of the second insulating layer20. The second connection terminals 22 are subjected to the surfacetreatment by the preflux treatment, the hot air leveling (HAL), theelectrolytic solder plating, the electroless solder plating, and thelike.

A description will be made below of a non-limiting method for forming aboard interconnection structure according to the third non-limitingembodiment while referring to FIG. 10A and FIG. 10B.

(I) First, solder plating 40 is formed for the first connectionterminals 15 and projection portions 16 of the first printed wiringboard 1 (refer to FIG. 10A).

(II) Next, the first connection terminals 15 and the second connectionterminals 22 are arranged so as to face each other (refer to FIG. 10B).

(III) Then, connection portions between the first connection terminals15 and the second connection terminals 22 are heated by the heater suchas the heater chip, whereby the solder plating 40 is molten, and theconnection layers 42 as shown in FIG. 9 are formed. The connectionlayers 42 are formed, whereby the first connection terminals 15 and thesecond connection terminals 22 are interconnected. When the solderplating 40 is molten to form the connection layers 42, the excessivesolder is stored in the side surface portions of the projection portions16, which are gaps between the first connection terminals 15 and thesecond connection terminals 22, thereby forming the fillets.

By the above-described manufacturing steps, the board interconnectionstructure according to the third embodiment, which is shown in FIG. 9,is formed.

In accordance with the board interconnection structure according to thethird embodiment, the excessive solder can be stored in the side surfaceportions of the projection portions 16, which are the gaps between thefirst connection terminals 15 and the second connection terminals 22.Accordingly, short circuit owing to the solder can be prevented.Moreover, since the fillets are formed on the side surface portions ofthe projection portions 16, the connection strength between the firstconnection terminals 15 and the second connection terminals 22 isenhanced. Specifically, with regard to each region surrounded by threesurfaces, which are: the surface of the first connection terminal 15;the surface of the second connection terminal 22; and the side surfaceof the projection portion 16, the surface area of the region concernedis increased, whereby a contact area of the region with the solder isincreased. Accordingly, the connection strength concerned can beenhanced.

Fourth Non-Limiting Embodiment

As shown in FIG. 11A to FIG. 11E, a method for forming a printed wiringboard according to a fourth embodiment of the present invention isdifferent from that of FIG. 8A to FIG. 8E, in the case of comparisontherebetween, in that projection portions 16 a are formed by halfetching. Other features shown in FIG. 11A to FIG. 11E are substantiallysimilar to those in the method for forming a printed wiring board, whichis shown in FIG. 8A to FIG. 8E, and accordingly, a duplicate descriptionwill be omitted.

A description will be made below of the method for forming a printedwiring board according to the fourth embodiment while referring to FIG.11A to FIG. 11E.

(I) First, the insulating layer 10, on a surface of which the conductivecircuits having the connection terminals 15 are arranged, is prepared.Then, the resist 50 is coated on the insulating layer 10 and theconnection terminals 15 (refer to FIG. 11A). As the resist 50, forexample, the photocuring photoresist can be used.

(II) Next, spots on which the projection portions 16 a are formed areirradiated with light and exposed, whereby the resist 50 is denatured soas to cure, and is formed into cured resist 52 (refer to FIG. 11B).Then, the resist 50 that has not turned to the cured resist 52 isremoved by an alkaline solution and the like (refer to FIG. 11C). Insuch a way, the resist 50 is patterned into a desired pattern.

(III) Next, the half etching is performed by using the cured resist 52thus patterned, whereby spots of the connection terminals 15, on whichthe cured resist 52 is not deposited, are thinned and turn to thinnedconnection terminals 15 a (refer to FIG. 11D). Meanwhile, the spots ofthe connection terminals 15, on which the cured resist 52 is deposited,are not changed in thickness, and accordingly, turn to the projectionportions 16 a.

(IV) Next, the cured resist 52 thus patterned is removed (refer to FIG.11E).

By the above-described manufacturing steps, the printed wiring boardaccording to the fourth embodiment is formed.

In accordance with the method for forming a printed wiring boardaccording to the fourth embodiment, the projection portions 16 a areformed by the half etching, whereby heights of the thinned connectionterminals 15 a and the projection portions 16 a can be made constant.

Fifth Non-Limiting Embodiment

As shown in FIG. 12A, a printed wiring board according to a fifthembodiment of the present invention is different from the printed wiringboard shown in FIG. 6 in that two projection portions 16 are provided oneach connection terminal 15. Moreover, as show in FIG. 12B, a boardinterconnection structure according to the fifth embodiment, which usesthe printed wiring board shown in FIG. 12A, is different from the boardinterconnection structure according to the third embodiment, which isshown in FIG. 9, in that a gap 60 is provided between each pair of theprojection portions 16. Other features shown in FIG. 12A and FIG. 12Bare substantially similar to those in the printed wiring board shown inFIG. 7 and the method for forming a printed wiring board, which is shownin FIG. 8A to FIG. 8E, and accordingly, a duplicate description will beomitted.

FIGS. 12A and 12B show that two projection portions 16 are provided oneach connection terminal 15; however, the number of projection portions16 provided on each connection terminal 15 may be a plurality that ismore than two. The projection portions 16 may be formed by plating orhalf etching. The plurality of projection portions 16 may be formed ofmaterials different from one another.

In accordance with the board interconnection structure using the printedwiring boards according to the fifth embodiment, the gaps 60 areprovided, thus making it possible to store the excessive solder in thegaps 60. Hence, the short circuit owing to the solder can be prevented.Moreover, the fillets are formed by using the gaps 60, thus making itpossible to enhance the connection strength between the first insulatinglayer 10 and the second insulating layer 20. Specifically, a surfacearea of each gap 60 surrounded by four surfaces, which are: the surfaceof the first connection terminal 15; the surface of the secondconnection terminal 22, and two side surfaces of the projection portions16, is increased, thus making it possible to enhance the connectionstrength.

Other Non-Limiting Embodiments

The description has been made as above of the present invention based onthe non-limiting embodiments. However, it should be understood that thedescription and the drawings, which form a part of this disclosure, donot limit the present invention. From this disclosure, variousalternative embodiments and application technologies should be madeobvious for those skilled in the art.

For example, in the first and second non-limiting embodiments, each ofthe cross sections of the first connection terminals 11 a and the secondconnection terminals 13 b forms the trapezoidal shape in which the sidesurfaces are linear; however, the side surfaces may be bent in the arcshape. If the side surfaces are bent in the arc shape, whereby thesurface areas of the side surfaces of the first connection terminals 11a and the second connection terminals 13 b are increased, then theconnection areas of these connection terminals to the solder areincreased, thus making it possible to enhance the connection strengththerebetween.

Moreover, in the third and fourth non-limiting embodiments, theprojection portions 16 are formed only on the first printed wiring board1; however, the projection portions 16 may be provided on the secondprinted wiring board 2. In this case, the projection portions 16 of thefirst printed wiring board 1 and the projection portions 16 of thesecond wiring board 2 are alternately arranged so as not to contact eachother, whereby the surface areas of the surfaces of the first connectionterminals 15, the surfaces of the second connection terminals 22, andthe side surfaces of the projection portions 16 are increased. Hence, ina similar way to the above description, the connection strength can beenhanced.

Furthermore, the description has been made such that each projectionportion 16 in the third embodiment is formed across the entire width ofthe connection terminal 15 in the direction where the conductive circuit14 is extended as show in FIG. 6; however, as shown in FIG. 13, eachprojection portion 16 may be formed to be shorter than the entire widthof the connection terminal 15 in the direction where the conductivecircuit 14 is extended. Each projection portion 16 is formed to beshorter than the entire width of the connection terminal 15 in thedirection where the conductive circuit 14 is extended, whereby the shortcircuit can be prevented by forming the fillet on an end surface of eachprojection portion 16 even if the molten solder flows in the directionwhere the conductive circuit 14 is extended. Moreover, the fillet isformed on the end surface of each projection portion 16, thus alsomaking it possible to enhance the connection strength between theconnection terminal 15 and the insulating layer 10.

As described above, it should be understood that the present inventionincorporates various embodiments and the like, which are not describedherein. Hence, the present invention is limited only by items whichspecify the invention in the scope of claims reasonable from thisdisclosure.

1-17. (canceled)
 18. A printed wiring board, comprising: an insulatinglayer; and a conductive circuit arranged on the insulating layer, theconductive circuit including, on an end portion thereof, a connectionterminal having an upper surface width that is narrower than a bottomsurface width thereof; wherein the connection terminal has a projectionportion formed thereon, the projection portion extending in a directionin which the conductive circuit is extended, and wherein the connectionterminal is provided on an end portion of the insulating layer orprovided near the end portion of the insulating layer.
 19. The printedwiring board according to claim 18, wherein the projection portion isformed of any one of copper plating, nickel plating, gold plating, and acombination of nickel and gold plating.
 20. The printed wiring boardaccording to claim 18, wherein a width of the projection portion is 75%of a width of the connection terminal.